The construction techniques used for a spiral staircase have developed from the use of wood where the pieces are cut and assembled at the site where the staircase is to be used, to factory assembled sections where these sections are assembled at the site. However the sections are heavy and difficult for a do-it-yourself home owner to assemble.
The sections usually consist of a single tread connected to an upright shaft, the treads are to be stacked forming a staircase. There is an angular rotation of each tread which forms a spiral pattern when stacked, as the staircase rises around the shaft. The staircase may wrap completely around as it ascends to the next level. These ascending treads are joined to each other by welding, nailing or bolting. In many cases there is a railing at the opposite end of the tread from the shaft.
Other manufacturers ship sections consisting of several treads already joined together. These sections are stacked at the site to form the staircase.
There are problems with setting the desired angular rotation for each step. For example, if the height to be traversed by the stairway is known and the rise and the angular position of one step to the next step is set by the manufacturer, then the position where the stairway terminates is fixed. Since the manufacturer sets these dimensions for the treads, the user cannot change these dimensions at the building site. The user is locked into the dimensions.
The preassembled treads are heavy and unwieldy since they are made of steel, concrete, or structural wood. Also if a section is damaged after the staircase is built, it is very difficult to replace or repair.
It is an object of this invention to form a lightweight modular tread design which can be stacked forming a spiral staircase, and where the tread can be formed as one piece.
It is a further object of this invention to create a flexible design so the angular rotation of the treads, as stacked to form the staircase, can be modified at the building site.
It is a further object of this invention to position the treads with a keying mechanism which locks the treads to one another.
It is another object of this invention to have a positioning and locking means which is fastened simply with a nut and bolt requiring little strength.
It yet another object of this invention to form a staircase which is inexpensive to manufacture, ship and build.